1. Field of the Invention
The present invention relates to a method for manufacturing a mold, and especially to a method for manufacturing a light guide plate mold using lithography technology.
2. Description of the Prior Art
A common type of liquid crystal panel needs a backlight module to provide uniform light and clear viewing of displayed images. A backlight module usually consists of a light source, a light guide plate, and a reflective plate. Light emitted from the light source is directed into the light guide plate, and is guided by the light guide plate and the reflective plate to emit from an emitting surface of the light guide plate. Then the light enters a diffuser, and the diffuser diffuses the light to provide uniform luminance for the liquid crystal panel. Usually, in order to attain uniform emitting light, some special optical structures are disposed on a bottom surface of the light guide plate to disperse what would otherwise be single internal reflections. The bottom surface is opposite to the emitting surface, and the optical structures are typically scattering dots.
Nowadays, manufacturing methods for a light guide plate are classified into two types: printing methods and non-printing methods. The printing method includes the following steps: printing some printing materials on a bottom surface of a light guide plate substrate, the printing materials being doped with highly diffusive materials such as SiO2 and TiO2; and heating and curing the printing materials. The diffusing materials are used to disperse what would otherwise be single internal reflections of incident light in the light guide plate. That is, the diffusing materials help scatter light before it is transmitted from the emitting surface, thereby providing uniform luminance for a liquid crystal panel.
The non-printing method includes the following steps: forming specific patterns on a surface of a light guide plate mold; injecting molten Polymethyl Methacrylate (PMMA) into the mold; and cooling the mold to obtain a light guide plate having desired specific patterns. The patterns are formed without any heating of a preform of the light guide plate. Thus the non-printing method has the benefit of lower cost.
In non-printing methods, mechanical tooling processes and lithography processes can be used to form the mold. Referring to FIGS. 7 to 11, a conventional lithography process is as follows:                1) sputtering a thin film 11 on a substrate 10, and coating a photo resist layer 12 on the thin film 11;        2) disposing a mask 13 having a predetermined pattern over the substrate 10, and exposing the photo resist layer 12 to a light source 14 emitting light passing through the mask 13;        3) developing the photo resist layer 12, and washing out a bare portion of the thin film 11 by using an erosive solution; and        4) stripping the residual photo resist layer 12 to form a pattern on the thin film 11, the pattern corresponding to that of the mask 13.        
Typically, the foregoing steps are repeated to produce a complex pattern on a light guide plate mold. The more complex the pattern is, the more repeats are needed, and the more masks having different patterns are needed. In particular, each of the masks has a tolerance, and the tolerances of the masks result in so-called “interferences” between the patterns of masks. That is, the precision and accuracy of the final pattern of the light guide plate mold is diminished. In addition, it is difficult to precisely control the results of washing out of the bare portions of the thin film by he erosive solution.